docs: def remove(self, participant):
          """
          Unsubscribe this participant from all topic to which it is subscribed.
          """
          for topic, participants in list(self._participants_by_topic.items()):
              self.unsubscribe(participant, topic)
              # It's possible that we just nixe the last subscriber.
              if not participants:  # IE, nobody is still listening at this topic.
                  del self._participants_by_topic[topic]